A significant need exists to detect personnel and vehicles that cross boundaries into forbidden areas. These boundaries may be the borders of a state or nation, or the perimeter around a facility. Nations have a need to detect attempts to cross their borders by agents of hostile nations or terrorist organizations, and by those who seek the economic or political benefit of residency without going through the lawful immigration process. Many facilities, with examples being airports, nuclear power plants, military bases, and penal institutions, are contained within perimeters that must be monitored to assure that no individual enters or leaves the facility without proper authorization. Timely detection of intruders will enable their interdiction by the forces charged with protecting the boundary.
Many territorial borders and facility perimeters run across irregular terrain that may include variations in elevation; drainage conduits typically referred to as washes or arroyos; boulders and rock formations; and foliage of various types. An effective intrusion detection system must include means to detect the passage of intruders who attempt to take advantage of these terrain features in their attempt to avoid detection.
Numerous intrusion detection systems have been developed that depend upon the generation of narrow beams directed parallel to the boundary to be protected. The passage of an intruder is typically detected by one of several means: by the interruption of one or more beams proceeding between a source and a receiver, by the reflection of transmitted energy in the beam back to a receiver collocated with the transmitter, or by detecting the infrared emissions from the person of the intruder. These intrusion detection systems typically depend upon the use of microwave, millimeter wave or infrared techniques that allow small, narrow beamwidth antennas or lens systems for practical deployment along a boundary.
A common problem with the beam breaker, radar, or infrared sensor intrusion detection systems is that they require an unobstructed line-of-sight between the sensor and the intruder for reliable operation. If uneven terrain, washes and foliage exist at numerous locations along the perimeter to be protected, these types of intrusion detection systems frequently fail to detect intruders.
An example of prior art that employs microwave beam interruption to achieve intruder detection and having some similarity to the present invention is disclosed by Kiss, U.S. Pat. No. 5,376,922, issued on Dec. 27, 1994. Two separately located microwave transmitter modules with directional microwave antennas, a sector module that includes two microwave receivers coupled to directional antennas that are deployed to receive maximum microwave energy from the two diversely located transmitter modules, and a remotely located central station comprise the basic elements of this intrusion detection system. The central station commands the transmitter modules to generate short coded emissions, the sector module receivers receive these signals, evaluate their characteristics, and determine if an intrusion has occurred based on a sufficiently large change in signal level. The central station is coupled to each of the transmitter modules and to each of the sector modules via UHF communication link to control operation of the modules, receive status information and detection data. Multiple transmitter modules and sector modules are deployed to form an intrusion detection barrier along a boundary. A single central station communicates with, and controls the operation of, the multiplicity of modules via a complex, multiple channel UHF system.
Another example of prior art that provides for the detection of intruders is disclosed by Gagnon, U.S. Pat. No. 6,424,259 B1, Jul. 23, 2002. This invention deploys a series of small patch antennas mounted at intervals along the vertical surface of a security fence or other similar structure. Spaced at a constant distance from the multiple patch antennas is a leaky coaxial cable installed along the surface of the ground. Microwave energy leaking from the cable develops an electromagnetic field in the area between the coax and the multiple antennas; alternately, emissions from the antennas are collected by the coaxial cable to form the electromagnetic field. Multiple switches are used to couple specific antennas and sections of the coaxial cable to a transmitter and receiver, and signal analysis equipment is used to determine if perturbations in the electromagnetic field have occurred in response to the presence of an intruder.
In contrast to the aforementioned prior art, the present invention operates in the upper UHF portion of the electromagnetic spectrum that allows both penetration of foliage and the detection of intruders. Microwave beams of the Kiss invention are typically blocked by foliage. The area between the fence-borne antennas and coaxial cable of Gagnon would be cleared of foliage, etc., in the process of installing the system to prevent obstruction of the microwave energy.
The present invention deploys multiple field disturbance transceivers (FDTs) with non-directional antennas in an array along a boundary to be protected. The array may also be deployed in two dimensions surrounding an object or area in need of protection. Essentially every FDT is identical to the others, rather than separate transmitter and receiver modules or complex methods to switch a transmitter and receiver to specific antennas or portions of a leaky coax as seen in the prior art. In the present invention, time division multiplexing and encoding of the transmissions allow each FDT to identify the source of every signal received and to relay data between FDTs. The FDTs transmit in a defined sequence along the array with only the first and last FDT communicating directly with a control station. In contrast, the invention of Kiss requires a UHF link between the central station and each module to accomplish control and receive information. Gagnon's system requires a complex arrangement to couple control signals to each of the switches that connect specific antennas and the coax to the transmitter and receiver functions. Other improvements, features and advantages of the present invention are described in the paragraphs to follow.